CID ⇝

Caller ID

(CID) A short piece of text transmitted by
some telephone systems describing the origin of a call,
e.g. the name of the caller. Some telephone handsets can
display this. A computer telephony integration system might
use it to trigger actions on the callee's computer such as
looking up the caller in a database and displaying their
details on screen.

There may also be a separate "caller id number" giving the
telephone number of the originator of the call.